Update Vulnerable Dependencies, Remove Deprecated Documentation Plugin, Update CI to include Java 21

[hnhv]

What does this PR do?

This PR updates the org.asynchttpclient:async-http-client dependency from 2.12.3 to 3.0.1, addressing a known vulnerability. In addition, it removes the deprecated org.ajoberstar.github-pages plugin and associated configuration from the build script.

Also updated the CI matrix to remove Java 8 and add Java 21, aligning with our support policy to run on Java SE versions currently under Premier Support.

We can explore a new approach (e.g., using git-publish) for automated documentation publishing in a future update. I will manually update the docs for this version.

Refactored SignatureUtil.java to replace deprecated Long(long) constructor with Long.toString().

Fixes #72

CHANGELOG

• [CHANGED] Updated org.asynchttpclient:async-http-client to 3.0.1 to address a vulnerability
• [CHANGED] Updated CI matrix to remove Java 8 and add Java 21, reflecting current support policy
• [CHANGED] Replaced deprecated Long(long) constructor with Long.toString() in SignatureUtil.java
• [REMOVED] Deprecated org.ajoberstar.github-pages plugin and associated configuration